A liquid crystal display element in a liquid crystal display device does not emit light itself. Thus, the liquid crystal display device includes a backlight unit arranged on the back surface of the liquid crystal display element functioning as a light source to illuminate the liquid crystal display element. In response to dramatic improvement of the performance of a blue light-emitting diode (blue LED) in recent years, a backlight unit using a blue LED as a light source has been in widespread use.
A light source using a blue LED includes as its components a blue LED to emit blue light and a phosphor to absorb light emitted from the blue LED and emit light of a color to become a complementary to blue (color including green and red or yellow). Such a light source is called a white LED.
A white LED converts electricity to light efficiently, so that it contributes effectively to lower power consumption. Meanwhile, the white LED has a problem of a narrow color reproduction range resulting from its wide wavelength bandwidth.
A liquid crystal display device includes a color filter inside a liquid crystal display element of this liquid crystal display device. Only a spectral range of red, green, and blue is taken out with the color filter for color reproduction. If a light source to be used has a continuous spectrum of a wide wavelength bandwidth such as the white LED, the purity of a color to be displayed through the color filter should be increased to expand a color reproduction range. Specifically, a wavelength band to pass through the color filter is set to be narrow. However, setting the wavelength band to pass through the color filter narrow in turns reduces light use efficiency. This is for the reason that setting the narrow wavelength band increases the amount of unnecessary light not to be used for image display by the liquid crystal display element. This reduces the brightness of a display surface on the liquid crystal display element, leading to a problem of increase in power consumption by the liquid crystal display device.
Using a semiconductor laser (or laser diode (LD)) as a backlight unit has been suggested as means to solve the aforementioned problems. The LD features a narrow wavelength bandwidth and excellent monochromaticity. By using the LD light source as a backlight, the color reproduction range of a liquid crystal display device can be expanded.
In a conventional system (direct-lit system) of placing a light source directly below a liquid crystal display device and irradiating the liquid crystal display device with light from directly below, using an LD as the light source enhances color reproducibility (color gamut) resulting from the high monochromaticity of the LD. However, compared to using an LED as the light source, using the LD results in a small angle of divergence of light to narrow a light-emitting region. Using the LD also causes a problem of nonuniform brightness that may be caused easily on a display surface.
The number of LDs to be used should be increased in order to solve the aforementioned problems caused by using an LD as a light source. However, this in turn results in a problem of increase in manufacturing cost and power consumption. An LD generates larger amount of heat than an LED. Additionally, an electric connection for driving the LD should be provided between the LD and a heat transferring member. These result in a problem of difficulty in thinning a heat sink for heat dissipation (see Japanese Patent Application Laid-Open No. 2011-76781, for example).
To compensate for the weakness of using a laser light source as a backlight occurring in the direct-lit system, the following system has been developed. According to this system, a light source is placed not directly below a liquid crystal display device but on a side surface (edge) of the liquid crystal display device and light is caused to shine on an entire display surface through a light guide plate to diffuse light. This system is called an edge-lit system (see Japanese Patent Application Laid-Open No. 2009-99316, for example).
The edge-lit system has an advantage in that it can reduce manufacturing cost and power consumption by reducing the number of light sources. Meanwhile, optical output from an LD changes largely as a result of the temperature characteristics of the LD. Thus, heat generated by the LD has not been dissipated sufficiently by a system of transferring this heat to a heat sink through a wiring substrate of the LD.
In the edge-lit system using an LD as a light source, the light source is arranged on a side surface of a panel. This allows thinning of a liquid crystal display device. However, large change in the optical output from the LD as a result of the temperature characteristics of the LD causes a problem that necessitates a highly-efficient heat sink.